TGF-beta's delay skeletal muscle progenitor cell differentiation in an isoform-independent manner

Satellite cells are a quiescent heterogeneous population of mononuclear stem and progenitor cells which, once activated, differentiate into myotubes and facilitate skeletal muscle repair or growth. The Transforming Growth Factor-beta (TGF-beta) superfamily members are elevated post-injury and their importance in the regulation of myogenesis and wound healing has been demonstrated both in vitro and in vivo. Most studies suggest a negative role for TGF-beta on satellite cell differentiation. However, none have compared the effect of these three isoforms on myogenesis in vitro. This is despite known isoform-specific effects of TGF-beta1, -beta2 and -beta3 on wound repair in other tissues. In the current study we compared the effect of TGF-beta1, -beta2 and -beta3 on proliferation and differentiation of the C2C12 myoblast cell-line. We found that, irrespective of the isoform, TGF-beta increased proliferation of C2C12 cells by changing the cellular localisation of PCNA to promote cell division and prevent cell cycle exit. Concomitantly, TGF-beta1, -beta2 and -beta3 delayed myogenic commitment by increasing MyoD degradation and decreasing myogenin expression. Terminal differentiation, as measured by a decrease in myosin heavy chain (MHC) expression, was also delayed. These results demonstrate that TGF-beta promotes proliferation and delays differentiation of C2C12 myoblasts in an isoform-independent manner.